Effects of lunar gravity on the muscles, bones, and joint capsules of the upper limbs of mice.

Abstract

Gravity (1 g) is crucial for musculoskeletal system development and maintenance. Reduced mechanical loading during spaceflight decreases muscle mass and bone mineral density in the lower limbs. However, its effects on the upper limbs remain poorly studied. In this study, we sought to elucidate the impact of lunar gravity on the muscles, bones, and joints of mouse upper limbs. Using the Mouse Habitat Unit aboard the International Space Station, male C57BL/6J mice (9-weeks-old) were assigned to either a Ground Control (GC) or a Partial-Gravity (PG) group. PG group mice were exposed to lunar gravity (1/6 g) for approximately 1 month during spaceflight and were euthanized shortly after return to Earth. Upper-limb samples provided by the Japan Aerospace Exploration Agency were used to evaluate the cross-sectional area (CSA) of triceps brachii fibers, cortical bone parameters of the radius and ulna diaphysis, and the thickness of the elbow-joint capsule. Compared with the GC group, the PG group showed no significant differences in cortical bone morphology, but exhibited a trend toward reduced CSA in the triceps brachii and a significant increase in elbow-joint capsule thickness. In the present study, no marked changes were observed in the muscles or bones of the non-weightbearing upper limbs; however, thickening of the elbow-joint capsule was detected. These findings suggest that exposure to a partial-gravity spaceflight environment is associated with joint capsular thickening; however, whether this effect is directly attributable to gravitational unloading or represents a secondary adaptation to altered activity patterns requires further investigation. These findings may highlight the need for and contribute to the development of strategies for maintaining normal upper-limb function during space missions.